GASTEROPODA. 



143 



Trochus Risso ; Phorcus Risso ; Cittarium PI til. ; Om- 

 phalitis Phil. ; Margarita Leach. 



Euomphalus. Soiv. — Shell orbicular, a planorbular 

 spire, with three or four vo- 

 lutions, imbricated above, 

 smooth below ; aperture of 

 a round polygonal form ; um- 

 bilicus large, penetrating to 

 the apex of the shell.* — 

 80 species ; fossil. 



Sowerby observes that 

 these fossils are very like Delphinula, the main difference 

 being that their whorls do not increase so rapidly in size. 

 Dr. Mantell has discovered that the inner whorls are 

 divided by partitions, as if the animal had retreated with 

 its growth from the apex of the spire; but these par- 

 titions are not perforated like the chambers of the Nau- 

 tilus. This part in the fossil is usually filled with spar.f 



Philippi has the following as sub-genera : — Serpularia 

 Romer ; Maclurites Lesueur ; Ecculiomphalus Portloek ; 

 Schizostoma Rronn, ; Centrifugus Hisinger. 



Rhaphistoma. Hall. — Shell compressed, conical, 

 with three or four whorls, which are not separated from 

 one another ; the umbilicus is moderately deep ; the aper- 

 ture almost triangular; the upper part of the whorls is 

 marked with a kind of keel, which arises from the lines 

 of growth turning in a different direction, which makes 

 the outer lip have a somew T hat shallow groove. — 3 species ; 

 fossil. 



Scalites. Conrad. — Shell seeming only to differ 

 from that of the last genus in the more elevated spire. 



* Sowerby's Genera of Shells. 



f The Medals of Creation. 



